ISA Server2004 and broadband connection

I am using broadband connection in my office and wants to share it with other users. I have installed Win 2003 server standard edition and joined it with domain. The broadband connection is using DHCP. The problem occur that when i installed ISA Server the external interface connected to broadband modem shows question mark on that (like this?). Please help me what configuration is required for ISA in this case.